The present invention relates to a control system for a power converter driving a synchronous motor and comprising a load commuted thyristor.
Wide-spread applications are found for power converters with a load commutated thyristor inverter, which utilize back electromotive force of the synchronous motor so that the main circuitry of the inverter can be simplified, and can be designed for high tension application, and for a higher efficiency. Accordingly, they are most suitable for variable speed drive of large-capacity motors.
As was described above, the back electromotive force is utilized for commutation. But, while the motor speed is low, the back electromotive force is not sufficiently high for the commutation. Accordingly, the commutation is effected by intermission of the current supplied to the inverter. For switching between the intermission commutation and load commutation, as well as phase control of the thyristors, a control system is employed.
A conventional control system has a disadvantage in that the torque of the synchronous motor varies sharply or rapidly when there occurs a switching from the intermission commutation to the load commutation. This rapid variation in the torque may cause vibration of the mechanical system, which in turn reduces the life time of the machine.